The White House was thrown on the defensive Thursday over President Trump’s choice to lead the Department of Veterans ­Affairs, forcing officials to fend off mounting skepticism that Ronny L. Jackson has the experience to run the government’s second-largest agency.

Trump announced by tweet late Wednesday that the White House physician would succeed ousted secretary David Shulkin, surprising veterans groups and lawmakers, who were not notified beforehand and scrambled to learn the policy views of someone whose positions on the chronic challenges facing VA are unknown.

Jackson is a career naval officer who was an emergency trauma doctor in Iraq before spending the past 12 years as a White House physician. But his résumé lacks the type of management experience usually expected from the leader of an agency that employs 360,000 people, has a $186 billion annual budget and is dedicated to serving the complex needs of the country’s veterans.

“It’s great that he served in Iraq and he’s our generation. But it doesn’t appear that he’s had assignments that suggest he could take on the magnitude of this job, and this makes Jackson a ­surprising pick,” said Paul Rieckhoff, chief executive of Iraq and Afghanistan Veterans of America.

Jackson was taken aback by his nomination, said senior White House officials, who spoke on the condition of anonymity to discuss internal deliberations. After aides gauged his interest in recent days, he hesitated to take on such a big job. But the president continued to push and told his senior staff Monday that the doctor was his top choice. A senior White House official described an informal interview process, without the extensive vetting that typically accompanies a Cabinet selection.

The White House planned to announce Wednesday that Shulkin would leave the administration and be replaced on an interim basis by Robert Wilkie, undersecretary for defense personnel and readiness at the Defense Department, until a nominee was found.

But Trump preempted the plan when he tweeted that he intended to nominate Jackson, administration officials said.

The active-duty rear admiral had been a behind-the-scenes figure while serving the past three administrations as a White House physician, but he moved into the spotlight in January when he delivered a glowing assessment of Trump’s physical and mental health to reporters, which aides said endeared him to the president.

The White House on Thursday defended Trump’s choice of Jackson, saying his hands-on experience as a doctor would serve him well as Veterans Affairs secretary.

“He knows what soldiers need on the battlefield and what they need when they come home as veterans,” deputy White House press secretary Lindsay Walters told reporters aboard Air Force One en route to Cleveland, where Trump delivered a speech on his infrastructure plan. “The president has full confidence in his pick and trusts he will be able to give veterans the care they deserve.”

Key congressional Republicans publicly took a cautious approach to the nomination.

“We are doing our homework on Dr. Jackson,” said Amanda Maddox, spokeswoman for Sen. Johnny Isakson (R-Ga.), chairman of the Senate Veterans’ Affairs Committee, which will hold Jackson’s nomination hearing. Trump called Isakson after announcing that he had picked the doctor to replace Shulkin, she said.

“His name was never floated around,” Maddox said, “so we are doing our due diligence.”

Trump’s decision to upend VA’s leadership comes as Senate Republicans were already worried about other potentially difficult nominations in the months leading up to midterm elections, when they want to focus their message on the recently passed tax cuts rather than deal with more upheaval in the administration.

“Any time Republicans are not selling the tax bill over the next seven months is a missed opportunity,” said GOP strategist Brian Walsh, a former spokesman for the National Republican Senatorial Committee. “I will say Senate Republicans are a little more insulated by the nature of the seats that are up. But there’s no question that these are unhelpful distractions.”

The stack of Trump nominees includes Gina Haspel, who was picked this month to be the director of the Central Intelligence Agency and is facing opposition from members of both parties because of her ties to the agency’s past use of brutal interrogation measures on terrorism suspects, which critics say amounted to torture.

Senate Republicans have told White House officials in recent days that the process of confirming CIA Director Mike Pompeo to replace Rex Tillerson as secretary of state is going to be challenging even though he is expected to be approved, according to two people briefed on the discussions. Democratic senators said privately when Pompeo was tapped to replace Tillerson that they expect far fewer Democrats to back him than the 14 who voted for him to lead the CIA.

Senior Senate Republicans have privately expressed frustration over the personnel battles that have raged since the beginning of Trump’s presidency and recently told the White House that they did not want to have to consider a series of nominees this year, according to aides and officials who have heard the complaints.

The move to dismiss Shulkin — as well as the lack of communication about Jackson — only fueled concerns on Capitol Hill that the administration was not doing enough to help Congress defend or even respond to the president’s rush of personnel changes.

Jackson’s policy views are unknown, particularly on the most pressing issue facing VA: how much access veterans should have to private doctors outside the system at government expense. Shulkin’s moderate views on the subject, which were at odds with many administration officials, helped end his tenure.

VA secretary is one of Washington’s most unforgiving jobs even for someone with extensive management experience. Shulkin, also a physician, had run large hospital systems — including VA’s — before taking the job. His predecessor, Robert McDonald, was a chief executive of Procter & Gamble. The secretary before him was a decorated retired Army general, Eric K. Shinseki, who was forced out after managers in the far-flung health system were found to have fudged waitlists for veterans’ medical appointments.

As recently as February, Jackson was a candidate to run VA’s health-care arm, the Veterans Health Administration, the country’s largest health-care system, with 1,200 hospitals and medical clinics. On the day of his interview, he told a selection panel that the president was unwilling to let him leave his White House job, according to two people familiar with the discussion.

The panel interviewed him informally anyway, asking him how he would drive change in such a large organization but not about his views on policy. One person who sits on the panel, and who spoke on the condition of anonymity because its proceedings are confidential, said they didn’t think Jackson had the requisite skills to transition from overseeing a team of about 20 doctors, nurses and physician assistants in the White House medical office to overseeing the health administration.

“I don’t remember him coming in trying to convince us he had the experience to do the job. He did not inflate his qualifications,” this person said. “The tone was, ‘Maybe I don’t have the same kind of experience as others who came before me in the job.’ ”

Jackson’s former colleagues in the Obama White House, who have publicly praised him in the past, said his nomination caught them off guard as they swapped text messages to ask how an extremely likable but unlikely candidate could end up running VA in the Trump administration.

“I’ve seen him managing a staff of a couple dozen, which he did to perfection,” said Ned Price, a National Security Council spokesman under Obama who recalled that he was treated by Jackson for a toe injury in the Philippines.

“But how that would translate to managing the second-largest department in federal government I have no idea,” Price said. “He has competence and integrity. I don’t think he’s going to fly around the world first-class or be buying thousands of dollars in furniture. But can he run VA? Anyone’s guess is as good as mine.”

Colleagues described the schedule of the White House physician as grueling, with continual foreign and domestic travel, always at the president’s side.

Some Democrats warned that if Jackson embraced the idea of privatizing more of VA’s health coverage, his nomination would be met with stiff resistance.

“I will carefully review Dr. Jackson’s qualifications to determine whether he has the best interests of our Veterans at heart or whether he, like many in the Trump administration, wants to push VA down the dangerous path of privatization,” Sen. Tammy Duckworth (D-Ill.), a wounded Iraq veteran, said in a statement.

At the American Legion, the country’s largest veterans organization, senior officials were putting together ideas to help Jackson acquaint himself with the agency and its challenges.